FullBypass : A Tool For AMSI And PowerShell CLM Bypass

FullBypass is a tool designed to circumvent Microsoft’s Antimalware Scan Interface (AMSI) and PowerShell’s Constrained Language Mode (CLM).

By doing so, it enables the execution of unrestricted PowerShell commands, providing a FullLanguage reverse shell.

While such tools can be used for legitimate penetration testing and security research, their unethical use is prohibited and may violate laws.

How FullBypass Works

1. AMSI Bypass:
   AMSI is a security feature in Windows that scans scripts for malicious content. FullBypass employs memory hijacking to neutralize AMSI’s defenses. Specifically, it rewrites instructions in the AmsiScanBuffer function using assembly-level manipulation.
   • By setting the size argument to zero with an XOR operation, AMSI becomes incapable of detecting subsequent PowerShell scripts or commands.
2. PowerShell CLM Bypass:
   PowerShell CLM restricts script execution to enhance security. FullBypass overrides this limitation, enabling unrestricted access to PowerShell’s FullLanguage mode.
   • This allows attackers or testers to execute advanced commands typically blocked in constrained environments.
3. Reverse Shell Creation:
   After bypassing AMSI and CLM, FullBypass prompts the user for an IP address and port number. It then establishes a reverse shell connection, granting remote access to the compromised machine in FullLanguage mode.

• Download the bypass.csproj file to a writable directory on the target machine (e.g., C:\Windows\Tasks or C:\Windows\Temp).
• Execute the file using msbuild.exe: textC:\Windows\Microsoft.NET\Framework64\v4.0.30319\msbuild.exe .\FullBypass.csproj
• The tool will bypass AMSI and prompt for IP/port details to establish the reverse shell.

While FullBypass demonstrates advanced techniques for bypassing Windows security features, its use must adhere strictly to ethical guidelines.

Unauthorized deployment of such tools is illegal and can result in severe consequences. Security professionals are urged to use it solely for authorized penetration testing or educational purposes within legal frameworks.